Ultradian endocrine rhythms are altered by a circadian mutation in the Syrian hamster

A single gene defect of the circadian clock (tau mutation) has recently been described that results in a shortening of the circadian activity cycle of the Syrian hamster. In the homozygous animal, free running activity is shortened by 4 h, resulting in a circadian period of approximately 20 h. Here, we examine the effect of the tau mutation on noncircadian oscillators by comparing the frequency of episodic secretion of LH and cortisol in normal period wild-type (~24-h circadian rhythm) and tau mutant (~20-h circadian rhythm) castrate females. Animals were ovariectomized at 14 weeks of age and maintained thereafter under conditions of constant illumination. Wheel- running records were obtained, and only those animals exhibiting clear single bouts of circadian activity were used in the experiment. Two days after intraatrial cannulation, blood samples were collected for a 5-h period every 5 min during the subjective day at the same relative phase of the circadian cycle. Deconvolution analysis revealed that LH pulse frequency was significantly reduced in the tau mutant females (33.3 ± 2.25- and 28.7 ± 2.0-min interpulse intervals for tau and normal period females, respectively). Cortisol pulse frequency also exhibited significant differences, with a reduced pulse frequency (32.8 ± 3.6- and 27.8 ± 1.4- min interpulse intervals for tau and wild-type females, respectively). There were no significant differences with respect to secretory pulse amplitude, hormone half-life or estimated burst amplitude, or mass of hormone secreted per burst for either hormone. We conclude that a genetic defect that affects the circadian clock located in the suprachiasmatic nucleus may have a more general effect on neural oscillators, including those controlling episodic hormone secretion.